This breathtaking oceanic giant is the world's heaviest bony fish. The Ocean Sunfish has a large blunt head, a small mouth with fused teeth forming a parrot-like beak, and tall slender dorsal and anal fins.

Instead of a caudal fin, Mola mola has a rudder-like structure called a clavus (meaning 'rudder' in Latin) – a deep, stiff lobe formed from extensions of the dorsal and anal fin rays. They also have a skeleton composed largely of cartilage, fewer vertebrae than other bony fishes, and lack pelvic fins, ribs, and a swim bladder.

The Ocean Sunfish grows to more than 3 metres in length, 4.2 metres in height, and can weigh more than 2.5 tonnes. Much of what is known about ocean sunfishes has come from stranded individuals.

Ocean Sunfish,
Mola mola(Linnaeus 1758)

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Distribution

The Ocean Sunfish is found in all tropical and temperate oceans, and occasionally comes inshore. The species has been recorded in Australian waters from off Fraser Island, Queensland, around southern Australia to about Mandurah, Western Australia.

Once thought to be a sluggish, sedentary species, Mola mola is actually an active swimmer, regularly diving to feed in depths below 200 m - and individuals have been recorded beyond 1000 m.

To warm themselves up after spending time in very cold deep water, Ocean Sunfish spend much of their time in the top 200 m of the ocean. They are often seen basking at the surface, flapping their dorsal fins out of the water.

Deep-bodied fishes, with high dorsal and anal fins, a reduction and fusion of caudal fin elements resulting in a loss of the caudal fin, a clavus or rudder-like structure, no swim bladder, no pelvic fins and a reduced, cartilaginous skeleton.The clavus is made up of fin rays and musculature from the dorsal and anal fins. The vertebral column is very reduced, and ribs are absent. Although the sunfish body is very rigid and rather inflexible, the large dorsal and anal fins are very powerful.Body depth 70–75% SL; head length 35–39% SL; visible broad band of reduced denticles (smooth to touch) alongside base of clavus from dorsal fin to anal fin.

The clavus is supported by about 12 rays, of which 8–9 bear ossicles. The ossicles are widely separated and less broad than the spaces between them. A visible band of reduced denticles (smoother to the touch than surrounding skin) is present at the base of the clavus extending from dorsal fin to anal fin.

Size

To 3.3 metres in length and 4.2 metres in height between the tips of the dorsal and anal fins.

Research using stable isotope analysis, has shown that groups of small ocean sunfish (less than 1 metre in length) feed on a broad range of pelagic organisms (Syväranta et al. 2012).

Once thought to be relatively inactive fishes that fed on pelagic gelatinous animals, research and sightings indicate that molas undertake deep-water forays to feed on colonial animals such as siphonophores.

Biology

Very little is known of the biology of ocean sunfishes. The sexes are separate and fertilisation is external. Individuals larger than 250 cm TL are female. Although some spawning areas have been identified and females may produce up to 300 million tiny pelagic eggs, Mola larvae are rarely collected.

Ocean sunfishes swim by synchronously flapping their dorsal and anal fins, using them like a pair of wings to glide through the water. This mode of swimming is thought to be very efficient, allowing ocean sunfishes to repeatedly dive to remarkable depths.

Fisheries

Although not a commercially important species, ocean sunfishes are frequently taken in extraordinary numbers as bycatch in gillnet, driftnet, longline and midwater trawl fisheries. Although not targeted, some fisheries catch more Mola mola that the target species.

Molids are used as a food fish throughout Asia and the largest markets are in Japan and Taiwan. They are also used in the traditional medicine industry. The marketing and sale of fish and fishery products derived from ocean sunfishes is banned in the European Union.

The targeting or incidental bycatch of ocean sunfishes is largely unregulated by fisheries around the world. Molas may also be 'finned' in some parts of the world, and may be threatened by floating debris such as plastic bags.

Remarks

Recent molecular work showed that there is high genetic divergence among Mola mola, indicating that there are two clades – one found exclusively in the southern hemisphere, and the other comprising individuals from both hemispheres.

Ocean Sunfish are host to at least 50 parasite species (Abe et al. 2012). Schools of Ocean Sunfish have been observed 'sun-baking' at the surface while gulls and albatrosses pick parasites from their huge bodies.

Similar Species

Etymology

Mola is from the Latin mola meaning "millstone", in reference to the disc-like shape of the body.